1. Field of the invention
This invention relates to a method of and apparatus for pyrolytically forming a silicon oxide coating on an upper face of a hot glass substrate.
2. Description of the Related Art
This invention was made as a result of research into various problems connected with the pyrolytic formation of silicon oxide coatings on glass. Silicon oxide coatings can be used either as sole coatings on glass for various purposes, or as one stratum of a multi-layer coating. For example, silicon oxide coatings may be used as subbing layers to be overcoated with other coating layers which may be of one or more different oxides or other materials such as metals, or as overcoating layers deposited on top of one or more such under-layers. The presence of a silicon oxide coating on soda-lime glass has the particular benefit of inhibiting the migration of sodium ions from the glass whether by leaching in the case of a sheet having no further coating, or by diffusion or otherwise into an upper coating layer either during formation of that upper layer or over the course of time. As an example, it has been found that in the pyrolytic formation of a tin oxide coating from stannic chloride on a soda-lime glass substrate, sodium chloride tends to become incorporated into the coating as a result of reaction of the glass with the coating precursor material or its reaction products, and this leads to haze in the coating. The presence of a silicon oxide undercoating or overcoating can also have a highly beneficial effect in reducing undesired interference effects due to variations in the thickness of the total coating.
The use of a silane, in particular SiH.sub.4, as coating precursor material is well known per se for the formation of pyrolytic coatings on glass. Silane decomposes at temperatures above 400.degree. C., and silicon coatings may be formed. It is difficult, however, to oxidise such a silicon coating in situ to form a silicon oxide coating. For this reason, it is preferable to react the silane directly with oxygen. In order that this reaction should take place to deposit silicon oxide on the glass substrate rather than on some part of the coating apparatus, all known proposals for the use of a silane-containing coating precursor material in the formation of a silicon oxide coating have insisted that the coating precursor material should only be allowed to mix with oxygen within a coating chamber which is open to the substrate to be coated, at a location where those materials are free to contact the substrate directly. We, however, have found that this is not favourable for the production of silicon oxide coatings of high and uniform quality, and in particular, there are problems in achieving a coating of uniform thickness across the width of the substrate.